Automotive diagnostic regulations require the engine cooling system to be monitored for achieving a predetermined coolant target temperature during a predetermined engine warm-up interval. According to the regulations, the thermostat is considered malfunctioning if the coolant temperature does not reach a manufacturer-specified target temperature within a manufacturer-specified period of time after starting the engine. It is apparent that the warm-up time interval is closely related to the driving conditions and, hence, the energy generated.
In the past, a strategy was developed to monitor the coolant temperature sensor for achieving a stabilized minimum temperature level which is needed for closed loop control operation. The strategy is a three region based logic, and each region is assigned a fixed warm-up time. It does not consider any driving conditions. If the strategy is applied to thermostat monitoring, the probability of false passes and failures will be high.
Accordingly, what is needed in the art is a method whereby the engine coolant warm-up can be monitored under a wide range of transient and steady state driving conditions while maintaining safeguards that prevent a false detection, either a false pass or a false fail.